In recent years, with the speeding up of transistors and the scale down of devices, a gate oxide film and the like are being made very thin. A gate of the transistor is generally formed in order of a well, a gate insulating film and a gate electrode. After the gate electrode is formed, side surfaces of the gate electrode are subjected to wet etching. Thus, the gate electrode is exposed, so that when a voltage is applied to the gate electrode, electric field concentration occurs at the exposed portion, causing a defect such as an increase in leak current. Therefore, an insulating film is generally formed on the exposed portion of the gate electrode.
Polysilicon is generally used as a gate electrode, but a metal having a low resistance value is laminated on it because the polysilicon has a high sheet resistance. As the metal to be laminated, a refractory metal such as tungsten or its silicide is selected considering workability and adhesiveness to a silicon oxide film and silicon itself. Where an insulating film is formed on side surfaces of the gate electrode exposed by etching, it is general to process by thermal oxidation at a high temperature of 800° C. or more.
But, tungsten is oxidized quickly at about 300° C., so that the resistance value of the tungsten layer increases when the gate electrode is subjected to the thermal oxidation. As a result, the resistance value of the gate electrode is increased. And, the tungsten and the polysilicon react to diffuse tungsten nitride (WN) of a diffusion preventing layer, possibly resulting in increasing a specific resistance.
The thermal oxidation requires a relatively long time. Therefore, it disturbs the improvement of productivity by increasing a throughput.
As a method of forming an oxide film other than the thermal oxidation, there is proposed a method of forming an oxide film by plasma as described in, for example, JP-A 11-293470 (KOKAI). It is a method for forming a silicon oxide film by introducing a silicon-containing gas and an oxygen-containing gas into a treating chamber, generating plasma of the gases, and depositing a silicon oxide film on a substrate, wherein hydrogen gas is introduced into the treating chamber in addition to the silicon-containing gas and the oxygen-containing gas to generate plasma containing hydrogen in the treating chamber. Thus, excellent film quality equal to that of a thermally-oxidized film can be obtained.
For the tungsten, it is desirable to perform processing at 300° C. or less to prevent oxidation, but for formation of an oxide film on silicon, a higher temperature is a preferable condition on the points that excellent film quality can be obtained, an oxidation rate is high, a difference in oxidation rate depending on a pattern density becomes small, and the like.